staging: comedi: ni_labpc: introduce labpc_ai_set_chan_and_gain()

Introduce a helper function to set the channel and gain bits in the
COMMAND1_REG.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>